1. Field of the Invention
The present invention relates to a heating roll for a toner fixing apparatus, for heating and fixing a toner-image onto paper in a copying machine, a printer, a facsimile machine, and so on, and particularly relates to an improvement in a heating resistor layer used for heating the heating roll.
2. Description of the Prior Art
Apparatus for fixing a toner-image onto paper in a copying machine, a printer or the like is known. Such apparatus can include a thermal fixing system in which resin in the toner is heated and melted to be thereby fixed or a pressure fixing system in which toner is pressed to be thereby fixed. Of the two, the thermal fixing system is widely used today since its stable fixing-ability can be maintained over a wide speed range from low to high, its thermal efficiency is high, and its safety is superior.
In a conventional heating roll used for the thermal fixing system, a halogen lamp is provided inside a hollow roll, or a Ni-Cr alloy resistance heating wire is provided on the circumferential wall of the roll. Recently, however, a variety of rolls having a distributed heating resistor layer around a cylindrical substrate have been proposed. Although a resistor layer including alumina and an Ni-Cr alloy is generally used as the distributed resistor layer, the specific resistance of the nickel-chrome alloy is so low that it is necessary to make the layer thin. It has proved to be difficult to fabricate a thin resistor layer have a uniform and predetermined resistance value.
In order to eliminate or reduce this disadvantage, there has been proposed a resistor layer including one or more kinds of oxides selected from alumina, magnesia and alumina-magnesia spinel, and a nickel-chrome alloy (Japanese Patent Unexamined Publication No. 61-134776). There also has been proposed a resistor layer which has a predetermined resistance value and which is easily produced to a proper thickness.
Such a proposed heating resistor layer includes ceramic maerials such as alumina, etc. at a considerably high ratio so as to provide a predetermined resistance value. However, the resulting heating roll is weak against bending, and therefore cracks can form in the heating resistor layer with the result that the predetermined resistance value cannot be maintained, particularly if repeated slight flexures occur during the tonerfixing operation. Moreover, in the case where a heating resistor layer is formed on the circumference of a cylindrical substrate, a mixture of ceramic materials and an Ni-Cr alloy usually is applied to the circumference by thermal spraying to thereby form the resistor layer. In this process, however, the amount of conventional metal alloy components of the resistor layer do not generate enough heat by oxidation to bond adequately to the cylindrical substrate, making it difficult to provide a heating resistor layer with a predetermined, controlled thickness.